Enhancement mode field effect transistors, for example MOSFETs (metal oxide semiconductor field effect transistors), are well known in the art, including various gating techniques. An enhancement mode FET conducts current between its drain and source in response to gate drive. The FET may be turned OFF by simply removing the gate voltage. Because of the capacitance that exists between the gate, source and drain of the FET, any change in the gate voltage is achieved only through an attendant movement of charge to and from the FET gate region. The speed with which a FET can be turned ON and OFF is dependent upon the speed with which the charge can be stored in and removed from the gate capacitance. Some gating circuits are able to supply sufficient current to charge the gate rapidly to attain fast turn-on, but must rely on a resistance connected between the gate and source of the FET to remove the gate charge for turn-off. In order to achieve fast turn-off a low value resistance must be used requiring that a high current be maintained through the gate resistance while the FET is ON, all as is known.
The present invention provides fast turn-off but without a high current required from the gate driving source.
The FET turn-off circuitry of the present invention is unpowered and uses a depletion mode JFET (junction field effect transistor) to drain off or deplete the residual stored charge in the gate to source capacitance of a MOSFET, in the preferred embodiment, upon removal of gate drive. Conduction of the JFET enables faster discharge therethrough of the MOSFET gate, whereby to facilitate faster MOSFET turn-off. The JFET is connected between the MOSFET gate and source, and is connected to the same gate drive terminal as the MOSFET gate. The JFET is pinched OFF in response to gate drive to prevent draining of the gate drive away from the MOSFET gate and hence insure turn-on of the MOSFET.